Northern Hemisphere Atmospheric Stilling Accelerates Lake Thermal

RESEARCH LETTER Northern Hemisphere Atmospheric Stilling Accelerates 10.1029/2019GL082752 Lake Thermal Responses to a Warming World Key Points: R. Iestyn Woolway1 , Christopher J. Merchant2,3 , Jamon Van Den Hoek4 , • Atmospheric stilling has resulted in 5,6 7 7 8 9 an increase in lake surface Cesar Azorin‐Molina , Peeter Nõges , Alo Laas , Eleanor B. Mackay , and Ian D. Jones temperature across the Northern 1 2 Hemisphere Dundalk Institute of Technology, Dundalk, Ireland, Department of Meteorology, University of Reading, Reading, UK, • A decrease in wind speed results in a 3National Centre for Earth Observation, University of Reading, Reading, UK, 4College of Earth, Ocean, and Atmospheric fi lengthening of the strati ed period Sciences, Oregon State University, Corvallis, OR, USA, 5Department of Earth Sciences, University of Gothenburg, and a strengthening of lake stability Gothenburg, Sweden, 6Centro de Investigaciones sobre Desertificación, Spanish National Research Council, Valencia, • Shallow lakes and those situated at 7 8 low latitude are influenced most by Spain, Chair of Hydrobiology and Fishery, Estonian University of Life Sciences, Tartu, Estonia, Centre for Ecology and atmospheric stilling Hydrology, Lancaster Environment Centre, Lancaster, UK, 9Biological and Environmental Sciences, University of Supporting Information: Stirling, Stirling, UK • Supporting Information S1 Abstract Climate change, in particular the increase in air temperature, has been shown to influence lake Correspondence to: thermal dynamics, with climatic warming resulting in higher surface temperatures, stronger stratification, R. I. Woolway, and altered mixing regimes. Less studied is the influence on lake thermal dynamics of atmospheric stilling, [email protected] the decrease in near‐surface wind speed observed in recent decades. Here we use a lake model to assess the influence of atmospheric stilling, on lake thermal dynamics across the Northern Hemisphere. From 1980 Citation: to 2016, lake thermal responses to warming have accelerated as a result of atmospheric stilling. Lake surface Woolway, R. I., Merchant, C. J., Van Den Hoek, J., Azorin‐Molina, C., temperatures and thermal stability have changed at respective rates of 0.33 and 0.38 °C/decade, with Nõges, P., Laas, A., et al. (2019). atmospheric stilling contributing 15% and 27% of the calculated changes, respectively. Atmospheric stilling Northern Hemisphere atmospheric also resulted in a lengthening of stratification, contributing 23% of the calculated changes. Our results stilling accelerates lake thermal fl responses to a warming world. demonstrate that atmospheric stilling has in uenced lake thermal responses to warming. Geophysical Research Letters, 46. https://doi.org/10.1029/2019GL082752 Plain Language Summary Studies of climate change impacts on lakes typically consider projections of air temperature over time. Such studies have demonstrated that a warming world will Received 9 MAR 2019 have numerous repercussions for lake ecosystems. Climate, however, is much more than temperature. In Accepted 3 SEP 2019 lakes, changes in near‐surface wind speed play an important role. Here, using a lake model to simulate Accepted article online 12 SEP 2019 the thermal behavior of lakes across the Northern Hemisphere, we show that lake warming has accelerated as a result of atmospheric stilling, the decrease in near‐surface wind speed observed in recent decades. Specifically, as a result of atmospheric stilling, lake surface temperatures have increased at a faster rate since 1980. Atmospheric stilling also resulted in a lengthening and strengthening of stratification, which is important for lake ecology and has numerous implications for lake ecosystems. Our results demonstrate that atmospheric stilling has influenced lake thermal responses to climatic warming and that future evolution of wind speed is highly pertinent to assessment of future climate change impacts on lake ecosystems. 1. Introduction Atmospheric stilling is the decrease of near‐surface (~10 m) terrestrial wind speed observed in recent decades (McVicar et al., 2012; Roderick et al., 2007). This slowdown has had impacts across the world, including regions where inland waters are present. Although the exact cause is unknown, some of the hypothesized drivers of atmospheric stilling include a reduction in the equatorial‐polar thermal gradient (McVicar et al., 2012), changes in mean circulation (Azorin‐Molina et al., 2014; Lu et al., 2007), and an increase in land‐surface roughness (Pryor et al., 2009; Vautard et al., 2010). Wind speed is one of the most important drivers of physical processes within lakes. Momentum and mechan- ical energy fluxes across the lake‐air interface scale as the wind speed squared and cubed, respectively (Wüest & Lorke, 2003). Modest fractional reductions in wind speed may cause substantial changes in stratification and mixing dynamics. Studies suggest that increasing air and, in turn, lake surface temperature typically, although not always (Tanentzap et al., 2008), leads to a strengthening of stratification, as a result of an ©2019. American Geophysical Union. increase in the temperature difference between surface and bottom waters. A concurrent decrease in wind All Rights Reserved. WOOLWAY ET AL. 1 Geophysical Research Letters 10.1029/2019GL082752 speed over lakes could reduce the magnitude of vertical mixing, leading to less heat being mixed from the surface to greater depths, and subsequently leading to an increase in surface temperature, a decrease in bottom temperature, and a strengthening of stratification. Such a process could accelerate the expected thermal impacts on lakes of climatic warming (Magee & Wu, 2017; Woolway, Meinson, et al., 2017). Alterations to temperature and stratification have profound effects on lake ecosystems. Increased surface temperature and more stable and longer stratification can favor bloom‐forming cyanobacteria (Paerl & Huisman, 2009) and influence lake productivity (O'Reilly et al., 2003; Verburg et al., 2003). Moreover, when a lake stratifies the deep water becomes decoupled from the atmospheric supply of oxygen and the longer the stratification lasts the more the oxygen becomes depleted due to in‐lake respiration (Rippey & McSorley, 2009), resulting in anoxic conditions and the formation of deep‐water dead zones (Del Giudice et al., 2018; North et al., 2014). This not only limits fish habitat for most species (Regier et al., 1990) but also alters the water chemical balance, promoting the production of methane (Borrel et al., 2011). Despite the potentially large decrease in wind mixing energy as a result of atmospheric stilling, the majority of climate change studies on lakes have ignored this influence, in part owing to an implicit assumption that surface air temperature is the dominant factor impacting lake thermal responses to climate change (O'Reilly et al., 2015; Winslow et al., 2018; Woolway, Dokulil, et al., 2017; Woolway et al., 2019). In this study, we aim to address this research gap by analyzing lake thermal responses to atmospheric stilling. We use a one‐ dimensional, numerical lake model to study the influence of atmospheric stilling on lake surface and bottom water temperatures, water column stability, and the number of stratified days per year in lakes situated across the Northern Hemisphere. 2. Methods 2.1. Study Sites The studied lakes were selected based on the availability of mean depth information as well as wind speed observations near lakes worldwide. Of the 1.4 million lakes globally (larger than 0.1 km2; Messager et al., 2016), only those situated within 10 km of a meteorological station were included in this study (n = 2,063). The majority of these lakes were situated in the Northern Hemisphere (n = 1,924), and thus, we restrict our study sites to this region. Of these 1,924 Northern Hemisphere lakes, not all were suitable for inclusion in this study. Lakes were deemed suitable if the lake surface area was considered large enough for the influence of terrestrial sheltering (e.g., tall tree canopy) on over‐lake wind speeds to be considered negligible, which depends on lake area and the sheltering height (e.g., local canopy height). According to field and wind tunnel experiments (Markfort et al., 2010), if the radius of the lake is 50 times greater than the average sheltering height (Read et al., 2012), we can expect terrestrial sheltering to have less influence on over‐lake wind conditions. In total, 1,123 lakes met these criteria. In addition, as we were interested in changes in thermal stability, lakes were only included if they stratified at any point seasonally, as determined from the lake model (see below). Following the recommendations of Balsamo et al. (2012) when using the selected model (see below) across a wide‐spectrum of lakes, we only included lakes in the analysis if their mean depth was less than 60m. This resulted in 650 lakes for the analysis. 2.2. Canopy Height Estimation The mean sheltering height of each lake was computed according to the land cover type within 250 m of a given lake perimeter following Van Den Hoek et al. (2015), where the lake shoreline polygons were extracted from Messager et al. (2016). Sheltering height was based on global canopy height data collected in 2005 using the Geoscience Laser Altimeter System (GLAS) lidar aboard the National Aeronautics and Space Administration (NASA) ICESat (Friedl & Sulla‐Menashe, 2015). Land cover type was based on the 2005 MCD12Q1 V6 Annual IGBP land cover classification product, derived from

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